The mechanism of activation of the epidermal growth factor receptor (EGFR) is being studied. A eukaryotic expression vector for the EGFR cDNA (long terminal repeat (LTR)/EGFR) has been engineered to direct the synthesis of this cDNA order Moloney murine Teckemia virus (Mo-MLV) LTR transcriptional control. Upon introduction of this vector into NIH/3T3 cells, the overexpressed EGFR was able to confer a conditional (EGF-dependent) transformed phenotype to the recipient cells. The high levels of EGFR necessary to confer this phenotype to NIH/3T3 cells were found in human tumor cell lines displaying amplification and overexpression of the EGFR gene. The LTR/EGFR vector was introduced into a naive cell line, the hematopoietic progenitor line 32D. In this cell, EGFR was able to confer responsiveness to EGF, which can now substitute as a mitogenic stimulus for interleuking-3 (IL-3), the physiological growth stimulus for 32D. The mitogenic dose-response analysis of 32D EGFR to epidermal growth factor (EGF) and the analysis of pathway activation (phosphatidyl-inositol (PI) turnover) by interleukin (IL-3) and EGF in these cells demonstrate that EGF is not likely acting through the IL-3 pathway but rather on an EGF- specific signaling pathway.